This invention relates generally to a printer of the type using a carriage translating laterally across the paper to be printed upon and more particularly to a control system for a serial printer where each line of print is begun at a uniform lateral position. In the prior art, many serial printers have been produced which use a step motor for driving a carriage. The output portion of the motor is provided with a detecting device to produce a signal indicative of the printing position for use as a timing signal when a printing head, carried on the carriage, is driven for printing. In printers of this type, the position at which printing begins is detected by ANDing the printing position signal with an output from a detecting device which allows a control circuit to detect a stand-by or home position of the carriage. In other words, coincidence between the home position signal and the print position signal is sufficient to permit the initiation of printing. However, the time period within which a change in level of the home position signal is detected, is equivalent to one interval between successive printing position signals, that is, one energizing interval of the step motor. Such a time interval or step is usually equivalent only to approximately 1/60 of an inch (0.423 millimeters). At the same time, the power transmitting mechanism from the motor to the carriage always produces vibrations because of variations in the applied voltage, changes in frictional loads, variation in conditions for driving the carriage, such as rotational speed of the motor, and other reasons. Additionally, because the power transmitting mechanism includes a spring factor which causes vibrations, and because clearances exist, the step motor and the carriage are subject to vibrations which differ in mode or phase with each other. As a result, the level of the home position signal may change at a time not within the predetermined period between particular printing position signals or within a predetermined energizing interval of the motor. Thereby, the position for start of printing is not uniformly detected by the control circuit. Thus, the printer begins printing at non-uniform positions. Further, as printers are operated at higher velocities or higher resolution characters are printed, these difficulties increase.
What is needed is a control system for a serial printer which insures a uniform starting position for printing on each line even at high speed operation and with normal vibrations occuring in operation.